Most hosts these days will look for ProcessDoubleReplacing first, and if the plugin supports it, send a double-precision audio stream. If the plugin doesn't support it, the host will call ProcessReplacing instead and send a single-precision audio stream.
A few hosts still support only single-precision audio streams, those hosts will only call ProcessReplacing. As bvesco correctly said, IPlug will convert the stream and forward calls from these hosts on to your plugin's ProcessDoubleReplacing.
On older processors, the single-precision processing will be faster. However in VST, if you want to support single-precision processing by default in a host that wants to do 64-bit processing, you have to *not implement* ProcessDoubleReplacing. IPlug won't do that unless you rewrite some code. On a post-P4 CPU, the processing speed difference between single and double precision is probably negligible.
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